How to rig a Seatbelt? (Buckle passing through seatbelt)
You could do it with Lattice:

Bring back your belt to a flat plane:



Create a lattice,  in the Properties panel &gt; Object Data choose its Resolution (here 2/1/5), in Object mode, scale it down, rotate it so that it frames the area that you need to tweak. Give your belt a Lattice modifier and choose the lattice as Object.



In Edit mode, move the vertices of the lattice so that it tweaks the belt the way you want. Parent the lattice to the buckle so that when you'll move the buckle, the lattice will follow and tweak the belt: